This invention relates generally to transistors and integrated circuits and more specifically to a horizontal structure transistor and method for making same.
It would be desirable for a bipolar transistor structure to easily lend itself to the fabrication of true complementary bipolar transistors. It would further be desirable that a process for fabricating such complementary bipolar structure be compatible with the fabrication of insulated gate field effect transistor (IGFET) structures on the same substrate. Further desirable features of bipolar transistors would include small size, easy automated layout by design automation tools, complete, improved isolation, and fast switching speeds.
Presently available transistor structures have several important inherent drawbacks. It is presently possible to make efficient, high gain vertical NPN transistors, but generally difficult or impossible to fabricate complementary PNP transistors having matching characteristics on the same substrate. Generally, when PNP transistors are desired on the same substrate as vertical NPN transistors, lateral PNP transistors are fabricated. It is well known that such PNP transistors are not truly complementary with vertical NPN transistors, and have several inherent limitations such as speed and gain. Additionally, even the relatively efficient vertical NPN transistors are wasteful of real estate on the semiconductor wafer, are difficult to isolate completely, and incorporate a number of undesired parasitic capacitances.
Additionally, it is presently difficult to fabricate bipolar and IGFET devices on the same substrate. To include IGFET devices on a single substrate with bipolar devices generally involves greatly increasing the number of process steps required, thereby decreasing yield and increasing cost.
It would be desirable for a transistor structure to overcome these and other limitations of the prior art. It would therefore be desirable for a new transistor structure to be small, easily susceptible to automated layout techniques, and have a process which is amenable to the incorporation of IGFET devices on the same substrate. Further desirable features of such a transistor structure include high gain, complete isolation, and low parasitic capacitance. Such features would result in vastly improved circuit switching speeds. It would further be desirable that the current carrying capacity of the transistor be easily controllable. Yet another important feature of such an improved transistor structure would be the ability to easily fabricate true complementary NPN and PNP bipolar transistors.
Therefore, an improved transistor according to the present invention consists of a horizontal structure transistor fabricated within an epitaxial island which is completely isolated. Such isolation may be achieved by forming the island on top of an an insulator, or within a layer of insulating material. The epitaxial island is completely surrounded by insulator. The isolated epitaxial islands are preferably shallow and of a regular shape, and can be extended in one dimension in order to improve the current carrying capacitry of the transistor. Horizontal transistors can be formed in such shallow epitaxial islands through suitable diffusions of base, emitter and collector regions. A polysilicon base contact rests on top of the island, and is available for contact with the remainder of the circuitry at, for example, one end of the isolated island.
Several preferred methods for fabricating such horizontal structure transistors are described below. Generally, the diffusions necessary for fabricating such transistors are compatible with the formation of IGFET devices on the same substrate, and doing so required the use of few or no extra mask steps. Fabrication of other devices, such as resistors and Schottky diodes, is also done within the isolated epitaxial islands.
Several methods for fabricating such isolated islands can be used. One preferred method involves the anodic formation of porous silicon which is then converted to silicon oxide. This process produces isolated islands of epitaxial silicon as desired, and these islands are suitable for the fabrication of transistors and other structures as described below.
The novel features which characterize the present invention are defined by the appended claims. The foregoing and other objects and advantages of the present invention will hereinafter appear, and for purposes of illustration, but not of limitation, several preferred embodiments are shown in the accompanying drawings.